1. Field of the Invention
The present invention relates generally to wireless communication devices, transmission power control methods therefor and wireless communication systems provided with the wireless communication devices and particularly to wireless communication devices capable of saving otherwise wastefully used transmission power to increase the longevity of a power source having a limited period for use, such as represented by batteries, transmission power control methods therefor and wireless communication systems provided with the wireless communication devices.
2. Description of the Background Art
Typically, wireless communication performs transmission with a fixed level of transmission power. In a wireless communication device, however, a battery or a similar power source having a limited period for use is used to supply power required for circuit operation and that for transmitting an electronic wave by wireless. As such, increasing the longevity of the power source is an important issue. To address this issue, various techniques have been proposed to refer for example to a distance for communication, a condition for communication and the like to control transmission power to save transmission power otherwise wastefully used.
One such technique is disclosed in Japanese Patent Laying-Open No. 9-93198. In this technique, a communication error rate is calculated for a predetermined period or amount of transmission or reception. If an obtained error rate is larger than a predetermined value, an infrared ray to be transmitted is increased in intensity to provide more reliable transmission. If an obtained error rate is smaller than the predetermined value, an infrared ray to be transmitted is decreased in intensity to reduce power consumption.
Another such technique is disclosed in Japanese Patent Laying-Open No. 10-13338. In this technique, a wireless communication device having transmitted data returns a response signal corresponding to an ACK signal (for normal reception) or an NACK signal (for abnormal reception) and when the ACK signal is received successively a predetermined number of times the current transmission power is decreased by a predetermined amount and whenever the NACK signal is received the current transmission power is increased by a predetermined amount to set the transmission power of the wireless communication device to be equal to a minimal level of transmission power receivable by a counterpart wireless communication device to minimize power consumption and thus extend a limited longevity of a power source.
The technique as disclosed in Japanese Patent Laying-Open No. 9-93198, however, requires calculating a communication error rate. As such, an infrared ray cannot have its intensity appropriately controlled for the predetermined period or amount of communication transmitted during the calculation of the communication error rate. As such, power is wasted for this predetermined amount of delay.
In the technique as disclosed in Japanese Patent Laying-Open No. 10-13338, a response signal is initially received and the determination of the ACK signal/NACK signal is then provided before transmission power is controlled. As such, if an NACK signal indicative of a reception failure is received from a counterpart wireless communication device and a retransmission of the same data is required, a period of time is required for a processing to increase transmission power before the data is retransmitted. This period of time is not negligible in rapidly transmitting data, disadvantageously resulting in a slow response.